OBJECTIVES: To determine wasting prevalence among infants aged under 6 months and describe the effects of new case definitions based on WHO growth standards. DESIGN: Secondary data analysis of demographic and health survey datasets. SETTING: 21 developing countries. POPULATION: 15 534 infants under 6 months and 147 694 children aged 6 to under 60 months (median 5072 individuals/country, range 1710-45 398). Wasting was defined as weight-for-height z-score <-2, moderate wasting as -3 to <-2 z-scores, severe wasting as z-score <-3. RESULTS: Using National Center for Health Statistics (NCHS) growth references, the nationwide prevalence of wasting in infant under-6-month ranges from 1.1% to 15% (median 3.7%, IQR 1.8-6.5%; ∼3 million wasted infants <6 months worldwide). Prevalence is more than doubled using WHO standards: 2.0-34% (median 15%, IQR 6.2-17%; ∼8.5 million wasted infants <6 months worldwide). Prevalence differences using WHO standards are more marked for infants under 6 months than children, with the greatest increase being for severe wasting (indicated by a regression line slope of 3.5 for infants <6 months vs 1.7 for children). Moderate infant-6-month wasting is also greater using WHO, whereas moderate child wasting is 0.9 times the NCHS prevalence. CONCLUSIONS: Whether defined by NCHS references or WHO standards, wasting among infants under 6 months is prevalent in many of the developing countries examined in this study. Use of WHO standards to define wasting results in a greater disease burden, particularly for severe wasting. Policy makers, programme managers and clinicians in child health and nutrition programmes should consider resource and risk/benefit implications of changing case definitions.